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Acquisitions and Divestitures - Drilling and Completion Costs Associated with Sale of Working Interests (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Drilling Carry Transaction Activities [Line Items]  
Cumulative proceeds from sale of working interest subject to drilling carry $ 559.5 [1]
Drilling Carry Recorded 386.5
Drilling Carry Remaining 613.5
Atinum MidCon I, LLC
  
Drilling Carry Transaction Activities [Line Items]  
Cumulative proceeds from sale of working interest subject to drilling carry 287.0 [1]
Drilling Carry Recorded 157.2
Drilling Carry Remaining 92.8
Repsol E&P USA, Inc.
  
Drilling Carry Transaction Activities [Line Items]  
Cumulative proceeds from sale of working interest subject to drilling carry 272.5 [1]
Drilling Carry Recorded 229.3
Drilling Carry Remaining 520.7
Mississippian Properties
  
Drilling Carry Transaction Activities [Line Items]  
Cumulative proceeds from sale of working interest subject to drilling carry $ 500.0
[1] Includes amounts related to the drilling carry.